1120S, Line 21, does flow to Sch K, Line 1.

Did you check whether you have entered any adjustments/override?  In the first instance, you may like to take a look at Ordinary Income > Schedule K > Income and Deductions, particularly on the line Ordinary business income (loss) [Adjust].  You may also like to type the amount you see on Sch K, Line 1, in the search box and see whether there is a hit under Field Data.

If no override/adjustment is found, there may be a problem with your return.  Your best bet then is to send the return to Support and have them look into that on the back end.